Added scons target test (scons test) which runs most of the unit tests.

Once 'scons test' becomes stable (after a few CI runs), it shall be fully
enforced through CI.

Following unit tests are covered with 'scons test'

controller/src/agent:test
    controller/src/vnsw/agent/cmn:test
    controller/src/vnsw/agent/diag:test
    controller/src/vnsw/agent/init:test
    controller/src/vnsw/agent/kstate:test
    controller/src/vnsw/agent/ksync:test
    controller/src/vnsw/agent/openstack:test
    controller/src/vnsw/agent/oper:test
    controller/src/vnsw/agent/pkt:test
    controller/src/vnsw/agent/services:test
    controller/src/vnsw/agent:test
    controller/src/vnsw/agent/uve:test
    controller/src/vnsw/agent/vgw:test
controller/src/analytics:test
    controller/src/analytics/ruleparser:test
controller/src/base:test
controller/src/bfd:test
controller/src/bgp:test
    controller/src/bgp/enet:test
    controller/src/bgp/evpn:test
    controller/src/bgp/inetmcast:test
    controller/src/bgp/l3vpn:test
    controller/src/bgp/origin-vn:test
    controller/src/bgp/rtarget:test
    controller/src/bgp/tunnel_encap:test
controller/src/control-node:test
controller/src/db:test
controller/src/discovery:test
controller/src/dns:test
controller/src/gendb:test
controller/src/ifmap:test
controller/src/io:test
controller/src/net:test
controller/src/opserver:test
controller/src/query_engine:test
controller/src/schema:test
controller/src/xmpp:test

Below tests have been commented out, as they don't pass [consistently].
Separate bugs shall be raised to fix these.

controller/src/config:test,
syslog_test,
bfd_server_test,
bgp_server_test,
bgp_stress_test,
bgp_xmpp_mcast_test,
rt_network_attr_test,
svc_static_route_intergration_test,
db_test,
db_base_test,
tcp_stress_test,
test_dummy,
test_uve,
xmpp_stream_message_client_test,
xmpp_stream_message_server_test,

Change-Id: Ia5e6c9f409b6182134bc777de40b0385ba2ac176

Comment out a flaky test

Change-Id: If0215a72151f4907089984cbc67aec6659afd871